This invention relates to a driving circuit of a fuel injector for an engine for controlling the fuel injector in accordance with an injection pulse signal and a method for controlling the same.
In general, in a fuel injection system for injecting pressurized fuel through a fuel injector of an electromagnetic valve to an engine, the fuel injector to be opened and closed at a high speed is required to expand a dynamic range. Thus, a driving circuit of the fuel injector is required to reduce an inductance and a resistance value of the fuel injector and to hence control a current in accordance with the inductance and the resistance value of the fuel injector.
Japanese Patent Application Laid-Open 57-203831 and Japanese Utility Model Laid-Open 60-47852 disclose, for example, a driving circuit of a fuel injector for controlling to initially supply a relatively large current to move a valve body of the fuel injector at a high speed against the static friction resistance when an injection pulse signal for designating an injection time of the fuel injector is ON and then to supply a small holding current necessary to hold an opening state of the fuel injector.
In this case, in order to control the current flowing to the fuel injector to a target value, a current detecting resistor having a small resistance value is connected in series with the fuel injection, a potential difference between both ends of the current detecting resistor is compared by a comparator with a control signal formed in accordance with an injection pulse signal by a logic circuit using various gates and comparators, and a transistor for driving the fuel injector is operated in accordance with a comparison output of the comparator.
However, a logic level of the signal formed by the various gates and the comparators has a width as known. A voltage level of the actual signal even in the case of a high level of the same logic "1" or a low level of the same logic "0" is different in accordance with an irregularity among types or individual elements of logic elements.
Therefore, even if the voltage level of the control signal becomes the logic level "0" in the comparator for comparing the potential across the current detecting resistor with the control signal on the basis of the injection pulse signal when the injection pulse signal becomes OFF to become a timing for stopping the fuel injector, the voltage level does not become completely "0", and the voltage level of the actual control signal might sometimes exceed the potential difference between both ends of the current detecting resistor.
More specifically, when the injection pulse signal becomes OFF to become the timing for stopping the fuel injector, a so-called offset voltage is generated at the control signal formed on the basis of the injection pulse signal, the transistor for driving the fuel injector is not completely shut off, but a small current flows to the transistor to cause the closing force of the fuel injector to become small, and hence the fuel might be leaked from the fuel injector.